Wells are used to recover a variety of substances from the ground. Typically, wells are merely vertical bore holes extending from ground level to the desired depth. Some means of extracting the desired material (typically a fluid) is then attached to or inserted into the well and used to draw the material out of the bore hole.
In many circumstances, the well is designed to extract a specific material which resides at a specific depth or narrow range of depths below the surface. For example, in extracting crude oil, the vertical bore hole will typically pass through an oil-bearing layer for a depth of about 6 to about 60 feet (about 2-18 meters). This range of depths from which oil can be most readily extracted is commonly referred to as the "pay zone". In extracting environmental contaminants, such as hydrocarbons released in a ground-level spill which have seeped into the ground, there tends to be a much shorter pay zone, with depths of 1-2 feet (30-60 cm) being fairly common.
Vertical bore holes may not always be the most optimal structures for recovering some materials, though. Underground oil-bearing formations can be discontinuous and can vary significantly in depth even within a given field. An idealized oil field structure, showing varying underground strata and oil-bearing formations O, is schematically illustrated in FIG. 1. As a result of the underground structure, sinking a single vertical bore hole requires some degree of luck in hitting a suitable oil-bearing formation. In the scenario illustrated in FIG. 1, the vertical bore hole V is located within a large oil field, but this bore hole misses the oil-bearing formations underlying the site.
To increase the likelihood of intersecting an oil-bearing formation, more complex bore holes have been drilled with a significant portion of the length of the bore hole extending generally horizontally. Such bore holes usually are formed using fairly conventional drill bits attached to a flexible, steerable shaft. In drilling these holes, a vertical bore hole is drilled. When the vertical hole is a predetermined distance (usually a number of feet) from the desired depth of the horizontal portion of the bore hole, a relatively viscous "mud" is urged out of a port located in a side of the housing of the drill bit. The pressure of this mud will urge the drill bit to the side, pushing it away from its vertical path. In this manner, the path of the drill bit can be curved so that it approaches a horizontal orientation. Once the drill bit is horizontally oriented, the goal is to maintain that orientation and extend the bore hole substantially horizontally.
Unfortunately, the control over such a "horizontal" drilling operation is not as precise as one might wish. For a given lateral fluid pressure exiting one of the side ports near the drill bit, the degree of curvature induced in the path of the drill bit will vary depending on the properties of the adjacent ground structure. As a result, it can be difficult to accurately determine the depth at which the drill bit will first reach a horizontal orientation. If the pay zone P is fairly large, that will leave sufficient leeway to accommodate such variances. Such variances can make this approach much less attractive if the pay zone is significantly smaller, e.g., in the case of recovery of groundwater contaminants, because of the difficulties in targeting such a specific depth.
Even if the correct horizontal level is initially achieved, though, it can be difficult to maintain. When attempting to drill horizontally with such a drill bit, underground formations of differing hardness will tend to deflect the bit from a horizontal path. As suggested in FIG. 1, the strata underground often are not horizontal. If a bit encounters an interface between a hard layer and a relatively soft layer, the drill bit will tend to follow that interface rather than continue on a horizontal path through the harder material. As a result of all of these difficulties, the path followed by the drill bit tends to deviate rather significantly from horizontal and may wander in and out of the targeted pay zone, as suggested by the path of the bore hole H in FIG. 1.
To counter these difficulties, horizontal drilling tends to be a rather complex, carefully controlled operation. The shaft must include several circumferentially spaced holes through which the pressurized "mud" can be forced to steer the bit in three dimensions. Expensive systems are used to track the position of the drill bit underground on a real-time basis. An experienced operator or a highly sophisticated computer controller must carefully adjust the pressure of the mud delivered through each hole in reaction to each deviation from the intended path. As a consequence, drilling horizontally tends to be exceedingly expensive.
If one has already defined a bore hole, any drill bit inserted into that same bore hole will tend to follow the same bore hole rather than deviate from the previously-cleared path. As a consequence, it can be problematic to try to drill a second horizontal bore hole from within a vertical bore hole created for a previous horizontal drilling operation. As a consequence, if one wants to try multiple horizontal paths across a single field, one usually has to sink a separate vertical bore hole for each and every horizontal path. This significantly increases costs, particularly for deeper pay zones.
Some wells may yield only a small volume of the desired material, limiting the commercial feasibility of the well. For example, some oil wells which initially produced substantial amounts of oil will having decreasing yields over time until only small amounts of oil can be extracted. It may even be necessary to use pumping aids, such as a so-called "horse-head" pump, to help pull the oil from the underground formation.
To increase the yield from such wells, it may be possible to fracture the surrounding rock and ease the flow of the desired material to the bore hole. In oil wells, this is most commonly done by injecting a highly pressurized fluid (e.g., water, oil or a gas) into the bore hole. It can be difficult to accurately direct this pressure and fracture the surrounding structure primarily in the pay zone, though, and the force tends to be dissipated non-beneficially in structure outside the pay zone.